At present, analog signals output from most of the infrared imaging systems and infrared detectors are turned into 14 bit digital signals through front-end analog-to-digital (AD) transformations, whereas the 14 bit digital signals are input into digital signal process modules to be transformed into 8 bit grayscale signals, and then the 8 bit grayscale signals are input into video decode modules to generate standard television (TV) signals so as to be output to monitors.
For turning the input signals of high bits and large dynamic scopes into 256 grayscale signals in conformity with the human eye's observation habits, a digital signal process module is used for performing processes of compress/stretch, overflow/cutoff, brightness shift and gain adjustment on digital signals rapidly and in real time, thereby the input signals of high bits and large dynamic scopes can be turned into image signals in conformity with the human eye's observation habits.
In one prior art system, based on a characteristic that there is usually consistency among current frame images, the accumulated average values of AD values (digitized energy values) of respective points in a last frame image are used as a reference of brightness shifting of a current frame image.
In another prior art system, based on the image equalization technology of column diagram statistics, a distribution curve is obtained by counting the number of pixels of respective grayscale levels in a last frame image, and an image is enhanced by compressing grayscale levels with no pixel numbers or very few pixel numbers and stretching grayscale levels with more pixel numbers when being output.
However, those two prior art systems have disadvantages as below.
For the first prior art system, when a bright target of small area whose brightness is much higher than an average value appears in a scene, the accumulated sum of AD values may be increased significantly, thereby the brightness of the current frame image may be decreased greatly, which even pushes other targets of large areas totally blank, therefore the observation for the target may be deteriorated.
For the second prior art system, while the image is enhanced, the background and noise are also elevated, such that even some “key targets” with small pixel numbers may be merged, and edge pixel points of the targets may also be merged, resulting in loss of image details.